The invention relates to an antenna diversity system for radio reception for motor vehicles, which comprises a multi-antenna system (2) having several antennas (A1, A2, . . . AN) with antenna feed lines 2a. There can be a diversity switching device for selection of a different reception signal, and an evaluation circuit which evaluates the reception quality of the reception signal just arriving at the receiver. This evaluation circuit is designed to bring a different reception signal in terms of diversity to the receiver if interference occurs, by switching over.
Antenna diversity systems having switching diversity of this type are preferably used for ultra-short-wave radio reception, and are known, for example from German Patent DE 19607045 and also U.S. Pat. No. 6,169,888 to Lindenmeier, the disclosure of which is hereby incorporated herein by reference. Furthermore, a diversity system is known from European Patent EP 1126631, which is also published as U.S. Pat. No. 6,925,293 the disclosure of which is hereby incorporated herein by reference in its entirety. That disclosure or patent aims at achieving a greater useful signal than with a single antenna, by means of same-phase superimposition of two or even more antenna signals, in order to thereby reduce the likelihood of level collapses in a territory with multi-path spread. With this, there is on average a more advantageous signal/noise ratio achieved in the sum signal, with reference to the receiver noise. However, perfect functioning of a same-phase summation of antenna signals is limited due to the fact that the partial waves that are superimposed at the reception location (Rayleigh reception field) differ only insignificantly in their momentary frequency. Using this design, there is no audible reception interference. In reception situations such as those shown in FIG. 1 in EP 1126631, in which wave bundles with different running times t0 to t3 are superimposed at the reception location, the received partial waves no longer have the same frequency and result in frequency interference swings, because of superimposition. These swings frequently result in interference that occurs spontaneously, after frequency demodulation while driving. The wave bundles having the different running times are superimposed at the reception location in accordance with a Rayleigh distribution, in each instance, which has different effects for the different antennas on the vehicle, so that the antenna signals of two diversity antennas on the vehicle can also possess different momentary frequency, particularly in the region of level fading.
The difference in these frequencies is caused by the frequency modulation of the high-frequency carrier and is generally very great, and the resulting phase difference would have to be regulated out in a signal path, by means of a phase rotation element, if the signal does not possess a different frequency interference swing in the other signal path. On the other hand, in case of fast phase regulation, a signal that experienced interference in the first signal path would impress its interference on the second signal path and therefore compel interference in the sum signal. Another disadvantage of a purely phase-regulated system is the restriction to two antenna signals, so that there is no sufficient diversity effect achievable with this system.
Adjacent channel interference acts in similar manner, due to a limited selection in the inter-frequency plane. Also, signals that occur in the reception channel due to intermodulation of other ultra-short-wave transmitters result in frequency swing interference on the useful signal, in combination with level collapses. This interference cannot be eliminated using the phase regulation system with the same phasing. To improve this situation, a controllable logic switching device is therefore contained in EP 1 126 631, in the multi-antenna system. With this design, a reception signal that is different in terms of diversity, in each instance, is passed to at least one of the two inputs of the reception device, with different switch positions, and the summed signal is passed to an interference detector for extremely rapid recognition of a sum signal that has experienced interference due to frequency interference swing. In this case, the interference recognition signal of this detector switches the logic switching device to a different switching position if reception interference is present.
However, the arrangement indicated in EP 1 126 631 has the remaining disadvantage of the occurrence of same channel or adjacent channel interference caused by undesired radio stations. These undesired radio stations frequently form a cause for interference, because of the close frequency occupation with stations. Thus, level maximization by means of the same phasing of the desired signal generally does not eliminate the interference phenomenon. Instead, in such situations it is important to improve the ratio of the useful signal to the interference signal. If the selection of a different reception signal, in terms of diversity, does not result in interference-free reception, the same phasing of the desired signal cannot lead to the goal, because generally the interfering same channel or adjacent channel signal will not be suppressed with this change. The same holds true for reception situations in which wave bundles having greater different running times are superimposed at the reception location. Another particular disadvantage of the arrangement indicated in EP 1 126 631 is in the practical implementation, which generally makes it necessary, for cost-advantageous implementation, for the phase regulation device to be accommodated in the receiver, and therefore at least two separate antenna lines must be brought to the receiver. In automobile construction, this means increased expense and added required space, and is classified as disadvantageous with regard to vehicle handling. As another disadvantage of the phase regulation device, there are, in the Rayleigh field, deep signal collapses, which the regulation device cannot follow, particularly when driving fast, and it must perform transient oscillation on them, thereby causing uncontrolled phase control with interference frequency swings to occur. This in turn can result in interference in reception, on the basis of the frequency demodulation.
However, the great expenditure of material, which results from the second high-frequency line to the receiver, in combination with the need to require a second tuner circuit in the receiver, for the diversity function, is particularly serious for the economic efficiency of the solution indicated in EP 1 126 631.